In general, the body of a smart card is made of plastics material. By way of example, the card body is formed by a stack of sheets of plastics material that are laminated together. The stack of layers comprises in particular a central layer, also known as an “inlay” layer, e.g. having the particular function of supporting the antenna.
Once the card body has been laminated, a cavity is machined in the body in order to form a housing for the microcircuit module. The cavity is defined by an inside surface presenting at least one area for making contact with the module in order to secure the module in the cavity when the module is received in the body. By way of example, the module may be fastened inside the cavity by adhesive, in particular adhesive bonding with said contact area.
At present there is a constant need to provide cards with new functions. It is thus known to add components to the card in addition to the antenna and the microcircuit module.
For example, it is known to add a light-emitting diode (LED) that is electrically connected to the antenna and that is suitable for lighting up to indicate that contactless communication is taking place properly.
The LED is generally formed by a component mounted on the surface of the support-forming inlay layer. Generally, this type of component is soldered onto its support by providing a mass of solder and then melting said mass in a “reflow” oven. The reflow technique serves to raise the solder to its melting point and thereby fasten the component. The reflow temperature is about 200° C.-230° C. and depends in particular on the nature of the solder.
Thus, the inlay layer needs to be made of a material that is capable of withstanding the high temperatures of reflow ovens, in particular temperatures that are higher than 200° C. By way of example, a plastics material that is suitable for this application is polyethylene terephthalate (PET).
The drawback of such a material is that it presents weaker adhesion with the adhesive materials that are used for fastening the module than does polyvinylchloride (PVC), which is the material that is conventionally used for fabricating other layers of the body. This raises a problem in particular when the cavity is formed at least in part inside the inlay layer and when the module, on being fitted inside the cavity, needs to be fastened in part to said layer.
Thus, when performing bending or twisting strength tests on a card, it is found that the module begins to separate after about one thousand cycles have been applied. This has the effect that a considerable number of smart cards need to be rejected because they present functional defects.